1. Field of the Invention
The present invention relates to a heat-transfer small size tube used for a heat exchanger in a refrigerator, an air conditioner, or the like, and a method of manufacturing the same.
2. Description of the Related Art
Recently, there have been strong demands for an energy- and space-saving heat pump type air conditioners with these demands, it is required to realize a highly efficient, compact heat exchanger as a main component.
In heat pump air conditioners, cross fin type heat exchangers are most frequently used. This cross fin type heat exchanger is manufactured in the following manner. Heat-transfer tubes are inserted in aluminum fins having louvers or the like formed in its surface to exchange heat with air, and a through hole formed therein to allow the heat-transfer tube to be inserted. Expansion plugs are then inserted into the heat-transfer tubes to expand the tubes, thus causing the outer surface of the heat-transfer tube to come into contact with the aluminum fin. The resulting structure is assembled in the main body of the heat exchanger, thus completing the manufacturing process. When the cross fin type heat exchanger is to be used, refrigerant such as Freon is fed into the heat-transfer tube.
Smooth tubes are used as conventional heat-transfer tubes. Recently, however, an inner grooved tube has been developed. This tube has a large number of fine spiral grooves formed in its inner surface with this tube, the performance of inside heat transfer coefficient has been improved, and hence the performance of heat exchangers have been improved. Currently, therefore, inner grooved tubes having outer diameters of 9.53 mm and 7.00 mm are mostly used.
Recently, there have been strong demands for more compact heat exchangers. In order to meet the demands, a compact heat exchanger effectively using heat-transfer tubes having an outer diameter of about 4 mm is being developed. Under the circumstances, the present inventors previously disclosed a heat-transfer small size tube in Published Unexamined Japanese Patent Application No. 62-98200.
A simple application of heat-transfer small size tubes, however, causes an increase in the inside pressure drop and makes no contribution to an increase in the performance of a heat exchanger. In order to more effectively use small size tubes, a high performance heat transfer small tube having optimized groove shape must be developed.
In addition, when heat-transfer tubes are expanded and assembled in a heat exchanger, ridges formed on the inner surface of the heat-transfer tube are deformed. If the wall thickness is constant, ridges on the tube inner surface deform more with a decrease in tube diameter, thus deforming the grooves. It is generally known that the groove depth greatly influences the heat transfer performance of a heat-transfer tube. Therefore, in order to improve the efficiency of a heat exchanger, the degradation in heat transfer performance due to deformation of grooves must be minimized.
In the manufacture of such inner grooved small size tubes, if an excessively narrow tube is processed by a method similar to the conventional manufacturing method, the tube may be broken in grooving process.
If, however, a grooving process is performed with respect to a tube having a very large outer diameter, and the tube is worked into a small size tube with a large diameter reduction rate, fine depressions 2 may be formed in the outer surface of a heat-transfer tube 1, as shown in FIG. 1, or split defects 3 on the metal surface are often caused on the outer surface of the heat-transfer tube. Therefore, this method is not suitable for the manufacture of this tube.